Open up ~/Library/Preferences/com.apple.iPod.plist and save a backup copy of it. Then, expand the tree to reveal "Family ID." Change that value to 10001, as shown above.

Save your changes. Launch Address Book and open the Preference pane. The Google contact sync option will be staring right at you.

If the name is any indication, this plist entry only exists if you've used an iPod with your Mac—and I have, on my test machine—so I'm not sure what happens for users who have never paired an iPod with their Macs. Feel free to let us know how it goes in the comments. Thanks, joshua ferris!